Abstract: The present invention is intended to provide a gasket which is able to be mounted on a plunger with a loosening preventing mechanism and, more specifically, a gasket including loosening preventing means provided for the plunger having a mechanism configured to prevent a joint portion between the plunger and the gasket from loosening, and a prefilled syringe preparation utilizing this gasket. The gasket is mountable on the plunger having a projecting portion in a groove of a screw portion for screwing the gasket thereon including a depressed portion with which the projecting portion is engageable when being screwed on the plunger provided on an inner periphery of the gasket.